Upon the fabrication of a semiconductor, a semiconductor device is formed by a lithography technique in which a resist is applied on the surface of a silicon wafer to form a photosensitive film, the film is exposed to light so as to form a latent image and the latent image is then developed to form a negative or positive image.
A negative resist composed of cyclized polyisoprene and a bisazide compound has heretofore been known as a resist composition for the fabrication of semiconductor devices. However, the negative resist involves a drawback that it cannot be accommodated to the fabrication of semiconductors integrated to high degrees because it is developed with an organic solvent, so that it swells to a significant extent and its resolving power is hence limited. On the other hand, positive resist compositions are believed that they can be satisfactorily accommodated to the integration of semiconductors to a high degree because they are excellent in resolving power compared with the negative resist composition.
Positive resist compositions composed of a novolak resin and a quinonediazide compound have been generally used in this field to date.
However, satisfactory results as to various properties such as sensitivity, resolution, rate of residual film heat resistance and storage stability are not necessarily obtained from the conventional positive resist compositions. There has been a strong demand for enhancing the performance of the positive resist compositions. In particular, the sensitivity is important to enhance the productivity of semiconductors and there has hence been a strong demand for making a positive resist composition highly sensitive. In order to attain such an object, various kinds of additive have been separately added to the positive resist compositions comprising the novolak resin and quinonediazide compound as base components. As examples of compounds added for improving the sensitivity as described above, namely, sensitizers, may be mentioned nitrogen-containing hetero-cyclic compound such as halogenated benzotriazole derivatives (Japanese Patent Application Laid-Open No. 37641/1983) and cyclic acid anhydrides (Japanese Patent Publication No. 30850/1981). However, the addition of these sensitizers is accompanied by problems, for example, that there is little difference in solubility between exposed portions and unexposed portions of the resist compositions, so that their rates of residual film are lowered and their resolution is deteriorated, and their heat resistance is reduced due to the plasticizing effect of the novolak resin, which is caused by the sensitizers.